The present invention is directed, in general, to cleaning semiconductor wafers and, more specifically, to an apparatus and method for detecting the wetness of a semiconductor wafer cleaning brush.
During semiconductor manufacturing, several processes create debris that must be removed from the semiconductor wafers to prevent any contamination of the integrated circuits (ICs) derived from the wafers. Some of the processes well known for depositing contaminating particles on the surface of semiconductor wafers are silicon polishing, laser scribing and chemical/mechanical polishing.
Silicon polishing is performed after a silicon ingot is cut into wafers to prepare the wafers for further precessing. Laser scribing is the process by which identifying numbers are scribed into the wafer, and chemical-mechanical polishing uses an abrasive slurry to planarize the wafer surface. Each of these processes creates debris that may cling to the wafer surface and present a potential contamination hazard. The most common particles left after such processes include tungsten, titanium, titanium nitride, aluminum, tantalum, polishing pad particles and slurry particles. With the high cost of semiconductor manufacturing and intense competition among manufacturers, every effort must be made to minimize the contamination hazard presented by one of more of these particles.
Thus, for reasons of both thoroughness and efficiency, these contaminants are perhaps best removed from the wafer surface by mechanical means. In a typical wafer cleaning apparatus, the surfaces of the semiconductor wafer are best cleaned of any residual debris by passing the wafer through a cleaning box having multiple rollers equipped with cleaning brushes rotating within. While in use, the combination of brush rotation and pressure applied to the semiconductor wafer through the brushes provides for the proper cleaning of the semiconductor wafer surfaces.
The cleaning brushes found in the cleaning box are usually constructed of polyvinyl alcohol (PVA) or a material having similar properties. Among these properties are the tendency of the cleaning brush to remain very hard when dry, but soft and spongy when kept wetted. Ammonium hydroxide or diluted hydrofluoric acid are common cleaning solutions used to wet the brushes while cleaning semiconductor wafers. In addition, the cleaning brushes may also be kept wetted with de-ionized water to maintain the soft, spongy surface found on wetted brushes when a cleaning solution is not needed.
However, a recurring problem in the art is the cleaning of a wafer with a drying or dried cleaning brush. This problem may occur when a new cleaning brush has been installed or when the cleaning apparatus is first used after a long respite where the cleaning brushes have been allowed to dry. Additionally, this problem may occur while the cleaning apparatus is in used if the brushes are not sufficiently wetted throughout the cleaning process. Since typical cleaning brushes used for semiconductor wafer cleaning become harder as they dry, significant damage to the surface of the wafers passed through a cleaning box with dry brushes may occur. Specifically, a dry, hard cleaning brush will easily scratch the surface of a wafer, often times damaging the integrated circuits (ICs) to be derived from the wafer. Sometimes the damage to the wafer is so extensive the entire wafer must be discarded. Thus, with the high cost of materials in the competitive semiconductor market, manufacturers cannot afford to risk passing a wafer through a cleaning apparatus without first being certain the cleaning brushes are adequately wetted.
Prior art efforts to determine if the brushes in a cleaning apparatus are sufficiently wetted before a semiconductor wafer is passed through have generally been inadequate. At first glance, an obvious solution would be to overly inundate the cleaning brushes with cleaning solution prior to and during the cleaning process. Of course, the expense of wasted cleaning solution leads most manufacturers to shy away from this approach. Another approach has been to put windows in the sides of the cleaning apparatus so that the brushes may be visually inspected before the cleaning process. Unfortunately, the most common cleaning solutions used in the industry are negatively effected by exposure to light. In some cases, exposure to light may even result in a chemical reaction in the cleaning solution decreasing its potency.
Another attempt to overcome the problem of dry cleaning brushes has been to incorporate a flow sensor into the chemical dispensing system of the cleaning apparatus. However, this approach is also often unsuccessful since it only informs the operator that a fluid is flowing through the dispensing system, but not whether the cleaning brushes have been sufficiently wetted by that fluid or even what type of fluid is being dispensed. Even if this approach is successful in guessing that the brushes are sufficiently wetted, there is no guarantee that the brushes will remain sufficiently wetted from one wafer to the next, throughout the cleaning process. Moreover, knowing whether a harmful solution rather than simply de-ionized water has flowed through the cleaning apparatus can be very helpful to prevent the risk of burning a technician who must open the apparatus to perform maintenance.
Yet another approach has been to physically open the cleaning apparatus to visually inspect the saturation of the cleaning brushes. One disadvantage to this approach is the time necessary for the technician to open the cleaning apparatus, make the inspection and then reseal the apparatus. A more serious concern is the risk of the technician being burned by any cleaning solutions present within the apparatus when opened. Of course, most manufacturers would like to avoid placing their technicians or other personnel at risk of being chemically burned by the cleaning solutions used during the cleaning process.
Accordingly, what is needed in the art is an effective technique for determining whether the cleaning brushes in a semiconductor wafer cleaning apparatus are sufficiently wetted with cleaning solution, before wafers are pass through the apparatus, that does not suffer from the deficiencies found in the prior art.
To address the above-discussed deficiencies of the prior art, the present invention provides a wafer cleaning apparatus. In an advantageous embodiment, the wafer cleaning apparatus includes cleaning brushes mounted within a brush box and a sensor associated with at least one of the cleaning brushes and configured to detect a degree of wetness of the at least one of the cleaning brushes. The sensor includes a number of configurations as discussed in detail below. For example, the sensor may be a compressibility sensor that is configured to detect the amount of force required to compress the cleaning brush. In most cases, the cleaning brushes are comprised of an absorbent material, such as polyvinyl alcohol, that becomes more compressible as the cleaning brushes become more wetted with a solution. Thus, a degree of compressibility can be related to a degree of wetness of a cleaning brush, which provides data that allows an operator to determine when the cleaning brushes are wet enough to send a wafer through the cleaning apparatus without incurring unnecessary damage.
Other embodiments include pressure sensors, optical sensors, torque sensors, pH sensor, humidity sensors, and acoustic sensors, all of which can be designed, i.e. configured, to detect a change in the amount of wetness of the cleaning brush.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.